1. Field of the Invention
Embodiments of the invention relate to a display device, and more particularly, to a liquid crystal display device and a method of manufacturing the same. Although embodiments of the invention are suitable for a wide scope of applications, it is particularly suitable for preventing signal delay within a gate driver and gate lines for a gate-in-panel (GIP) type.
2. Discussion of the Related Art
Generally, liquid crystal display (LCD) devices, as flat panel display devices, change arrangement of liquid crystal molecules, which are initially arranged along a predetermined direction, by applying an electric field to the liquid crystal molecules to control transmittance of light passing through a liquid crystal cell to thereby display images. The liquid crystal cell is fabricated by arranging two substrates such that transparent electrodes of the substrates face each other, interposing a liquid crystal layer between two substrates, and sealing an injection hole. Polarizing films are attached on outer surfaces of the substrates. The light transmittance of the liquid crystal cell is controlled by the intensity of a voltage applied to the electrodes such that the liquid crystal cell displays graphic symbols/images by an optical shutter effect.
FIG. 1 is a flow chart illustrating a fabrication process of the related art liquid crystal cell for an LCD device. As shown in step st1 of FIG. 1, a first substrate and a second substrate are prepared. Thin film transistors and pixel electrodes connected to the thin film transistors are formed on the first substrate. A color filter layer and a common electrode covering the color filter layer are formed on the second substrate.
As shown in step st2 of FIG. 1, first and second alignment layers are formed on the first and second substrates, respectively. The first alignment layer covers the pixel electrodes, and the second alignment layer covers the common electrode. This step includes forming a polymeric thin film by coating the substrates with polymer and rubbing the polymeric thin film. The polymeric thin film should be deposited substantially over the entire surface of the first and second substrates with a uniform thickness and should be uniformly rubbed.
The rubbing method includes rubbing the alignment layer along the specific direction by a rubbing cloth, and has advantages such as easy orientation treatment, suitability to mass production, high stability of the orientation and easy controllability of a pre-tilt angle. Other methods, including photo-alignment, are also possible. An organic material of polyimide series is mainly used as the alignment layer, and a rubbing method is mainly used as the aligning method of the alignment layer.
As shown in step st3 of FIG. 1, a seal pattern, which forms a gap for liquid crystal material injection and prevents leakage of the liquid crystal material, is formed on one substrate. The seal patterning process involves forming a desired pattern by application of a thermosetting resin. A screen print method using a screen mask and a seal dispenser method using a dispenser are used for the seal patterning process. For the simplicity of fabrication, the screen print method has mainly been used. However, since the screen mask is not suitable for a wide substrate and a contamination by contact between the mask and the alignment layer often occurs, use of the seal dispenser method has gradually increased.
As shown in step st4 of FIG. 1, a spacer having a specific size to maintain a precise and uniform gap between the first and second substrates is deposited by spraying the spacer onto one of the first and second substrates. The spacer spray method can be divided into two different types: a wet spray method that involves spraying a mixture of alcohol and spacer material and a dry spray method that involves a spraying spacer material alone. The seal pattern and the spacer may be formed on different substrates. For example, the seal pattern may be formed on the second substrate, which has a relatively flat surface, and the spacer may be formed on the first substrate, which functions as a lower substrate.
As shown in step st5 of FIG. 1, the array and color filter substrates are arranged and attached by pressure-resistant hardening of the seal pattern. The aligning accuracy of the substrates is decided by a margin. If the substrates are attached beyond the margin, there is leakage of light, to thereby decrease the quality of images of the liquid crystal cell. Therefore, the aligning accuracy of several micrometers is required.
As shown in step st6 of FIG. 1, the attached liquid crystal substrate is divided into unit cells. A cell cutting process includes a scribe process that forms cutting lines on a surface of the substrate using a diamond pen or a cutting wheel of tungsten carbide, a hardness of which is higher than a hardness of the glass substrate, and a break process that divides the unit cells by force.
As shown in step st7 of FIG. 1, a liquid crystal material is injected into the unit cells. The unit cell has an area of several cm2 and a gap of several micrometers. To effectively inject the liquid crystal material into the unit cell, a vacuum injection method using pressure difference between the inside and outside of the unit cells is commonly used as an effective injection method. Since fine air bubbles included in the liquid crystal material can deteriorate the display property of the unit cells, a bubble-eliminating process, in which the cells are kept in a vacuum state for a long period of time, is required.
After finishing the liquid crystal material injection, an injection hole is sealed to prevent leakage of the liquid crystal material. Generally, a ultra violet (UV) curable resin is deposited onto the injection hole by use of a dispenser and then ultra violet light is irradiated onto the resin, thereby hardening the resin and sealing the injection hole. The unit cell is then tested. After that, polarization films are attached on outer surfaces of the unit cell, and a driving circuit is connected to the unit cell using an attachment process.
Recently, a gate-in-panel (GIP) type LCD device has been suggested and developed. The GIP type LCD device, in which a gate driver is formed on a substrate of a liquid crystal panel, decreases manufacturing costs and minimizes power consumption.
FIG. 2 is a schematic view of a GIP type LCD device according to the related art. As shown in FIG. 2, a liquid crystal panel 1 includes a display area AA for displaying images and a non-display area NA surrounding the display area AA. In the display area AA, gate lines GL and data lines DL cross each other to define pixels P, and each pixel P includes a thin film transistor T, as a switching element, and a pixel electrode (not shown) connected to the thin film transistor T. The thin film transistor T switches on/off according to signals of the gate line GL and electrically connects the pixel electrode and the data line DL. The pixel electrode forms a liquid crystal capacitor Clc with a common electrode. The liquid crystal capacitor Clc is connected to a storage capacitor Cst.
A gate driver 2 is formed at a side of the non-display area NA, and a data driver 3 is attached at another side of the non-display area NA. The gate driver 2 sequentially provides gate-driving signals to the gate lines GL, so that the pixels P connected to one of the gate lines GL are selected. Whenever the gate lines GL are sequentially selected, the data driver 3 provides RGB data signals to the data lines DL. The data signals are provided to the pixels P, and an electric field is induced between the pixel electrode and the common electrode. The electric field varies according to the data signals. The transmittance of light passing though a liquid crystal layer is controlled by changing the electric field, and thus images are displayed.
In the GIP type LCD device, the gate driver 2 is formed on a substrate of the liquid crystal panel 1 through the same processes as the thin film transistor T of the pixel P. That is, at the step st1 of FIG. 1, elements of the gate driver 2 are formed when the thin film transistors and the pixel electrodes are formed on the first substrate. The data driver 3 may or may not be formed on the substrate of the liquid crystal panel 1.
FIG. 3 is a block diagram schematically illustrating a gate drive of a GIP type LCD device according to the related art. In FIG. 3, the gate driver 2 includes a shift register composed of N (N is a natural number) stage circuits. The stage circuits are driven by 4 clock signals.
A first stage circuit receives a first clock signal CLK1 and a starting signal VstN and outputs a first gate-driving signal Vout1 to a first gate line (not shown). A second stage circuit receives a second clock signal CLK2 and the first gate-driving signal Vout1, as a starting signal, and outputs a second gate-driving signal Vout2. A third stage circuit receives a third clock signal CLK3 and the second gate-driving signal Vout2, as a starting signal, and outputs a third gate-driving signal Vout3. A fourth stage circuit receives a fourth clock signal CLK4 and the third gate-driving signal Vout3, as a starting signal, and outputs a fourth gate-driving signal Vout4. A fifth stage circuit receives the first clock signal CLK1 and the fourth gate-driving signal Vout4, as a starting signal, and outputs a fifth gate-driving signal Vout5. At last, an Nth stage circuit receives a clock signal CLKm (m is one of 1 to 4) and an (N−1)th gate-driving signal (not shown), as a starting signal, and outputs an Nth gate-driving signal VoutN.
Each stage circuit of the gate driver 2 for the GIP type LCD device includes thin film transistors. Channel widths of the thin film transistors of the stage circuits are several hundred times larger than the thin film transistors formed in the display area AA of the liquid crystal panel 1 of FIG. 2. A parasitic capacitance may be induced between the thin film transistor of the gate driver 2 and a second substrate in the non-display area NA. The parasitic capacitance is larger than that in the display area AA. The parasitic capacitance lowers characteristics of the thin film transistor and affects the gate-driving signals Vout1 to VoutN to thereby cause incorrect operation. In addition, the fabrication process for the liquid crystal cell of FIG. 1 is carried out as the gate driver 2 is formed in the non-display area NA, and thus particles can go into the gate driver 2. Because particles may go into one of the thin film transistors of the gate driver 2, the thin film transistor may be shorted with others, and one stage circuit, which is connected to the shorted thin film transistor, may be totally turned OFF.